The present invention is related to a contact connection between a semiconductor chip and a substrate, in particular between a memory module chip and a memory module board and to the method of producing the contact connection. While the principles of the present invention are applicable to any desired semi-connection between a semiconductor chip and a substrate, the present invention and the problems on which it is based will be explained with reference to a memory module chip and a memory module board.
Contact connections in chip-on-board technologies may generally be implemented by using either solder connections or adhesive bond connections. Primarily, memory modules are fitted with a large number of individual chips, for example 32, in order to implement a desired module memory size. Although the individual chips are tested in advance and defective chips are removed, it may occur that the interplay of the individual chips on a module leads to functional failures. These failures are caused inter alia, by different signal propagation times in the individual chips. In this case, an individual chip or several chips on the module board must be replaced by new chips. For this purpose, the contact connections between the individual chip and module board must be detached again and the contact surfaces of the module board must be prepared for new contact connections for each chip being replaced.
In the case of the soldered connections the defective chip is heated locally above the melting temperature of the solder and removed from the module board by pulling. This process, which uses a lead-containing solder, is the most widespread one in chip mounting technology. With regard to environmental compatibility, however, attempts are being made to change over to a lead-free solder compound in the future. However, lead-free solders have the disadvantage of a higher melting point temperature which leads to the chip being subjected to considerable thermal loading during mounting. This thermal loading leads to a increased failure rate (retention fails) and therefore to a lower module yield.
Adhesive bonding connections can be cured at low temperatures that are less polluting. They will therefore increasingly replace solder connections in the future.
Adhesively bonded connections can be produced with either thermoplastic or thermosetting adhesives. Thermosetting adhesives can be detached again only under a considerable tensile force, even when heated above the glass temperature. In addition, adhesive residues can be removed from the module board only with great effort. Although thermoplastic adhesives can be easily detached when heated above the glass temperature, their adhesive capacity in the cured state is generally too low to ensure reliable connection between the chip and module board.
The production of memory modules therefore requires a possibility of detaching individual defective chips from the module board again and replacing them by new chips. The existing repair processes predominantly use polluting lead-containing solders or, in the event that the lead-free solders are used can be carried out only at elevated temperatures. Thermosetting adhesives are difficult to remove and thermoplastic adhesives are too unstable.